Cathode ray tube



P. T. FARNSWORTH 2,149,045

CATHODE RAY TUBE 12, 1935 2 Sheets-Sheet 2 Feb. 28, W39.

Original Filed March EXCITING ATTORNEYS.

Patented Feb. 2 8, 1939 UNiiED STATE carnonn nav 'runn Philo '1.Farnsworth, Springfield Township, Montgomery County, 'Pa., Television &Radio Corporation, Dover, a corporation of Delaware application March12, 1935, 19,604. Divided andtliis application rear, Serial no. 138,921

assignor to Farnsworth Del.,

Serial No.

3 Claims. (01. 250-175 This'application is a division of my applicationentitled Means and method for interrupting electron multiplication",Serial No. 10,604, flied March 12, 1935.

I My invention relates to electron multipliers, namely, to means andmethod for causing small space currents to liberate large numbers ofadditional electrons to permit relatively large proportional spacecurrent to flow, and pmticularly, it relates to a means and method forremoving certain limitations in the operation of electron multipliers,disclosed and claimed in my copending application, Serial No. 692,585,filed October 7, 1933; of which the present application is animprovement. v A

In the application above cited, the theory and practical aspects ofelectron multiplication by secondary emission are discussed, pointingvout therein that certain limitations. in the maximum multiplicationobtainable could be overcome by interrupting the action periodically. Itis with V I this phase of electron multiplication that the presentapplication deals, together with the circults and preferred embodimentsof the apps.-

V ratus for that purpose.

Among the objects of my invention are:. To provide means for causing asmall number of electrons to initiate a relatively large proportionalelectron flow; to provide a television image dissector of greatlyincreased sensitivity; to provide a space charge device of novel typehaving char-- radio receiving device; to provide a multiplier operatingintermittently; to provide current multiplication of a high degree; toprovide an electron multiplier and circuit therefor adapted for highoutput currents; to provide a means and method of obtaining electronmultiplications of exceptionally high values; to provide an electronmultiplier of exceptionally small size having high current outputs; toprovide an electron multiplier operating without an external focusingfield; to provide an electronmultiplier which is self-interrupting inits action; to provide a means and method. for interrupting the actionperiodically of an electron multiplier in order to remove factorslimiting multiplication; and to provide a. new and novel method ofoperating electron multipliers. I y

My invention possesses numerous other objects and features of advantage,some; of which, to gether with the foregoing, will be set forth in thefollowing description of'speciiic apparatus em-' bodying and utilizingmy novel method. It is, therefore, to be understood that my method .isapplicable to other apparatus, and that I do not limit myself, in anyway, to the apparatus of the present application, as I may adopt variousother 5 apparatus embodiments, utilizing the method, within the scope ofthe appended .claims.

Referring to the drawings: v

Figure 1 is a view, partly in section and partly in elevation of themultiplier end of a preferred form of television dissector tubeembodying my invention. w i

Figure 2 is a circuit diagram reduced to lowest terms, showing themultiplier-dissector tube of which a portion'was shown in Figure 1,connected 1!! for use in television or similar service.

Figure 3 is a sectional view taken through the multiplier of the tube ofFigure l, as indicated by the line 3-4 in Figure l I Figure 4 is acircuitishowing an embodiment of my invention as applied to radioreceiving.

Figure 5 is a circuit diagram showing another form of multiplierconnected in a circuit where the action is periodically interrupted.

Figurefi is a. sectional view taken as indicated by the line 6-6 inFigure 5.

Figure '1 is a circuital diagram of another embodiment of my inventionas applied to radio receiving.

Figure 8 is a partial sectional view of the multiplier used in Figure'7, taken as indicated by the line 88 in Figure '7.

The present invention. considered broadly. employs certain apparatus ofmy copending appllca-. tion referred to above, more particularly, anelectron multiplier. The multiplier broadly comprises a chamber soevacuated that the mean free path of electrons therewithin is at leastseveral times the dimension of the chamber so that no appreciableionization will be produced by electrons making a traversal thereof. Thecathodes within the chamber are defined by a-pair of opposed plateswhich may, indeed, betermed cathodes since their mean potential isnegative and. since they are used under certain conditions of operationfor the emission of electrons.

Positioned between the, cathodes is an anode or collecting electrodewhich is maintained at a potential positive to the mean potential of thecathodes and which is so shaped," positioned or both that it isimprobable that an electron traversing a path betweenthe cathode will becollected thereby. Improbable" is here to be understood in itsmathematical sense with the corollary. that an electron makingsumcienthum-ace ber of traversals wfll certainly be thus collected. Theimprobability may be increased by establishing within the chamber aguiding field which tends to hold the electrons in a path which avoidsthe anode, or decreased by collector electrode construction. In thepresent method, I may use the mutual fields of the electrodes themselvesto create this guiding field, or I may'use a field externally applied.

Where the device is used to multiply an external photoelectric current,an aperture is preferably provided in one of the cathode plates, and aphotoelectric cathode is positioned without the chamber and thedischarge is directed through the aperture.

The operation of the device is based upon electrons within the chamberoscillating back and forth between the plates and releasing additionalelectrons in the chamber by repeated impacts. While there are a numberof methods by which this may be accomplished, these methods difi'eringsomewhat in their circuital requirements, all of which are explained andset forth in my prior application, which may be referred to for detailedtheory, I shall here describe only the first of these methods, as thisapplicationmore particularly concerns the operation of the device in thefollowing manner, although modifications of the method and apparatus mayeasily be made by those skilled in the art to encompass operation inother manners within the scope of the appended claims.

Electrons are directed towards the cathode and multiplication occurs bysecondary electron emission therefrom. A relatively high frequencypotential, which may be of the order of 60 megacycles, is appliedbetween the cathode plates, this potential being preferably relativelysmall as compared with the collecting potentials on the anode. Under theinfluence of the cathode energization, electrons strike one or the otherof the cathodes and emit secondary electrons which are acceleratedtowards the opposite cathode by the anode potential. If the potential ofthe latter be so related to the frequency applied to the cathode thatthe released electrons travel the space in time to be accelerated by theoscillating potential on the opposite cathode, a further impact andrelease of secondaries will occur, and if the ratio of secondaryemission. be greater than unity, a multiplication will take place whichwill increase until the number of'electrons released at each impact isequal to the number collected by the anode, or until the process isstopped by changing the anode potential or otherwise.

Two other factors serve to limit the available multiplication. The firstof these is the space charge which develops when the number of releasedelectrons becomes very large. This charge tends to drive the peripheralelectrons, namely, the electrons more remote from the center of thecloud traversing the tube, toward the anode, making their collectionthereby more probable. The second factor is the transverse component ofthe electrostatic field within the chamber.

When the electrons strike the opposite plate with sumcient velocity tocause emission of secondary electrons, the emitted secondaries areaccelerated in the opposite direction to generate new secondaries at theplate or cathode where the first electron was emitted, and if the ratioof secondary emission be greater than unity, a

multiplication by this ratio will occur at each impact. The anodepotential contributes only to the mean velocity of the electrons throughthe tube and has no direct eifect whatever on the velocity of impact,since the acceleration it imparts to the electron leaving one of thecathodes is exactly neutralized by the deceleration imparted to the sameelectron approaching the other cathode. A' change in mean velocity will,of course, vary the multiplication by changing the ratio of the transittime to the period of oscillation.

Although the collection of any individual electron by the anode isimprobable owing to the shape and position of the latter, and to thepresence of the guiding field, a certain proportion of the totalelectrons will be collected. This proportion will depend upon theportion of the cathodes which are emitting secondaries, namely, uponwhether the electrons are striking near the center or near the edges ofthe cathodes; upon the transverse component of the electrostatic fieldwithin the chamber, as determined by the space charge, the curvaturebetween the lines of force between cathode and anode; and upon any biaswhich may be applied within the tube.

Eventually, however,- a point will be reached where the number of newsecondaries emitted is equal to the number collected at each impact andthe current in the anode circuit will become constant.

Within certain limitations, therefore, the less the probability of anyindividual electron being collected, the greater the equilibrium currentwill be; and hence, this current will be increased by strengthening theguiding field. A limitation to this is, however, the space chargedeveloped when the number of electrons in the cloud which travelsbetween the plates becomes very dense, causing saturation.

Up to the point of saturation, the output of the device variesproportionately to either the number of electrons supplied to thechamber when used as a multiplier of electrons supplied from theoutside, or to the value of the externally applied alternating voltageon the cathodes when starting from stray electrons. In the latter case,the number of trips is varied, while in the first case, the same numberof trips is accomplished but the cloud is initiated by a differentnumber of electrons. In either case, how ever, the saturation limits areapproached when the multiplication is made large.

Current multiplication, however, can be obtained with this apparatus bylimiting the average number of impacts resulting from a single initialelectron so that the total output current remains below the equilibriumvalue. The mode of operation with which this particular application isin general concerned, comprises broadly, interrupting the multiplieraction periodically at such intervals that the limiting conditionscannot supervene. As these intervals preferably will include the samenumber of half cycles, and hence, the same number of multiplyingimpacts,it is clear that the main output current between the interval will beproportional to the number of initiating electrons liberated or createdin the interval.

The periodical interruption can be obtained in a number of differentways, for example, such as energizing the cathodes from one source ofalternating poten ial at a predetermined frequency and interrupting theaction of the tube at a lower frequency. By using high excitingfrequencies, a very small multiplying structure is made possible, smallenough, in fact. to be incorporated inside of a photoelectric tubeadapted for television or similar apparatus. By such a combinationsuitable gains of one hundred thousand to one million are easilyobtained and experimentally a current input of an interrupted multiplierof amperes gave a stable and usable output current of from .1 to 1million:-

pere.

In certain other cases, I may prefer to energize the cathodes directlyand solely by a modulated signal and interrupt the action to obtain highmultiplication. I may also prefer to cause the deviceto oscillate and tointerrupt itself to obtain the same result. Furthermore, I am able toutilize various structural modifications in the device and, for example,by winding a fairly open mesh grid around the collecting anode, toincrease the probability of collection. I am also able to operate thedevice with a guiding field created solely, by the relative sizes andshapes of the electrodes of the device. Furthermore, I prefer to makethe two cathodes of such shape that they substantially describe acylinder and by closing the ends of the cylinder, I am able to removeinterference with the multiplication due to the ionization'of thesecondary emitting ma-,

terials.

Having thus described the general theory of the multiplier in its broadsense, I now wish to describe my present modifications thereof asexemplified by the preferred embodiments illustrated herein.

As one of the uses to which my invention is ideally adapted is themultiplication of electrons emitted from a photoelectric or similarsource, I preferto describe one embodiment of my invention as forming afunctional part of a television dissector tube such as has beendescribed and claimed by Farnsworth Patent No. 1,773,980, and byRutherford in his application, Serial No. 696,999, filed November 7,1933. Such a structure utilizing my preseniinvention is shown in Figures1, 2 and 3.

In a preferred embodiment modified to include one use of the presentinvention, a cylindrical glass blank i is provided with mounting arm 2on which is supported, through the medium of the usual stem 4, aphotoelectric cathode 5. While I have shown this cathode being ofsomewhat concave shape, it may be planar if desired, the shape beingmerely toreduce distortion in scansion, as will be pointed out later.The cathode itself may well be formed of silver and be photosensitizedby the deposit of caesium thereon. in ways well known in the art. Theopposite end of the blank is closed with preferably a flat glass endwall 6 through which a light beam may be projected by a lens I in orderthat an optical image of an object may be focused on the cathode 5.

Just-inside of the end wall 6 is positioned a multiplier assembly whichis shown in enlarged detail in Figure 1. The multiplier is composite andcomprises a glass tube 9, one. end of which engages a tube positioningarm IE on one side of the blank. The other end of tube 9 is closed. andhas an anode ii sealed therethrough projecting along the axis of thetube, this anode being outwardly extended to pass through an anode stemii on the other side of the blank, the tube extending preferablydiametrically across the blank. The sealed end of the tube is maintainedin place by an anode sleeve i3 which engages the sealed end of themultiplier tube and also engages the end of the anode stem I2.

Thus, the multiplier assembly, being engaged with both sides'of theblank is maintained in position. I prefer to extend the sleeve along thetube and to provide it with a sleeve aperture i4 opening towards thephotosensitive cathode. The

anode sleeve l3 has an energizing connection I! the fllm adjacent themultiplier assembly.

I also prefer to so evaporate this him that it will not be present overthe front face 6 of the blank so no light will be excluded, but ,whichwill extend along the blank to contact the oathode 5. I do not, however,wish to make this coating continuous between the multiplier end of thetube and the cathode end and therefore prefer to shield oil a zonearound the envelope during evaporation or otherwise remove the coatingsubstantially intermediate the two ends of the blank to form aninsulating barrier band I! so hat a cathode film ill will be separatedfrom the multiplier film is.

The glass tube 9 contains a complete multiplier assembly. Thismultiplier comprises a pair of opposed cathodes 2i and 22. Thesecathodes are preferably separated portions of a cylinder, a space 24being left between them, and I prefer to form the cathode 2! with acomplete cylindrical end portion 25 having an aperture 29 thereinthrough which a glass sleeve 21, which is sealed around the anode Ii,extends. This sleeve 21 serves to position the end of the oathode 2i andmaintain it in position. The opposite ends of cathodes 2| and 22 areformed to substantially close that end of the cathode assembly so thatin effect the combination of the two cathodes describes a cylinder withsubstantially closed ends. The split in between the two cathodes ispreferably at right angles to the longitudinal axis of the blank so thatcathode 22 is facing towards the photoelectric cathode I and cathode 28is facing away from photoelectric cathode 5.

A target aperture 29 is provided in the glass tube immediately belowsleeve aperture H, which is directed towards the photoelectric cathode Sand immediately inside of target aperture 29 is a smaller cathodeaperture 30 which acts as the scanning aperture of the multiplier.Thetwo cathodes 2i and 22 are preferably sensitized so that they canreadily emit secondary-electrons at a rate greater than unity whenproperly impacted by electrons in motion, sensitization by caesium, forexample, being conveniently performed through a sensitizing tubulation,the remains of which are shown as a tubulation seal 3! on thepositioning extension 50. For such sensitization, the cathodesarepreferably made of silver.

In the modification shown in'the drawings. the anode sleeve l3 extendsabove, the therefore would surround, if completely cylindrical, themultiplier cathodes 2| and 22. These cathodes .are designed to carryhigh frequency, as will be capacity between the sleeve l3 and themultiplier cathode 2| is then utilized for purposes later to beexplained.

The two cathodes areconnected by means of a resonating coil 32preferably of silver wire positioned inside the glass tube, one end ofwhich is connected to cathode 2| through a coil connection 33 stabilizedby cathode insert 34 and an axial connection 35 is connected at one endto the cathode 22 and is stabilized by another cathode insertion 36,this cathode connection 35 extending axially through the silverresonating coil 32 and making a connectingweld 31 therewith at the outerend. The axial connecting wire then extends on out through an end sealI9 so that outside connection may be made to the cathodes and to theresonating coil 32.

It is quite convenient, due to the fact that good secondary emission isobtained by the deposit of caesium, to make not only both cathodes 2iand 22 of solid silver, but also make the resonating coil 82 and thecathode connection 35 of the same material.

Up to a certain point, the operation of the dissector tube is the sameas that of the prior dissector tubes referred to above.

An optical image is fogused from an object through objective lens I ontothe photoelectric cathode 5. This cathode will'then emit photo electronsin proportion to the intensity of the light falling on each elementaryarea. The electrons are accelerated towards the multiplier end of thetube by means of a positive anode potential supplied by an acceleratingsource til which is connected between the cathode I and the associatedfilm I8, and the anode sleeve it with its 4 cillated in two dimensionsover the aperture by the magnetic fields developed by suitable scanningcoils l8 and 46, excited by oscillators 41 and 40 respectively, whichpreferably generate scanning waves of saw-tooth form. All of theelementary areas of the electron image are thus successively traversedacross the aperture to accomplish the scanning of the image.

It will be seen that the total magnetic field, compounded of thefocusing and the two deflecting fields, varies as the image isdeflected. Since the distance from the cathode at which the electronsfrom any given elementary area of the cathode are brought to a focusvaries inversely as the total strength of the magnetic field and alsoinversely as the electron velocity, the focal surface tends to vary fromthe plane of the aperture as the electron image is deflected, movingcloser to the cathodeat the instant of maximum deflection and fartheraway as the deflecting fields approach zero.

The electrode structure, comprising cathode, anode, and the connectedfilms l8 and I8, com pensate for thiseffect. In first place, the film l9being in contact with the wall of the tube I adjacent the window 8,electrons directed toward the junction of film and window strike theglass with sufficient force to cause it to emitsecondaries, leaving apositive charge on the glass,

which increases and spreads progressively until the entire window is atthe anode-film potential, and the electric field distribution within thestructure becomes the equivalent of one due to two cup-shaped electrodesplaced mouth to mouth andseparated" by the gap iii. In the absence ofthe magnetic fields, this field distribution would serve toconcentratethe cathode discharge in a small circle surrounding the aperture, inaccords ance with the now well known principles of "electro-staticfocusing" or "electron-optics".

The magnetic fields overcome this effect, spreading the beam out into anelectron image of substantially the same size as the optical image andof high definition, but the non-uniformity of the electrostatic fieldhas another and more important effect which is not affected by themagnetic fields, and that is to vary the mean velocity with which theelectrons from the various parts of the cathode traverse the tubes. Allof the electrons have the same velocity upon their arrival at the anode,but those traveling from the periphery of the cathode towards theaperture receive more of their acceleration in the first part of theirJourney than do those leaving from the center of the cathode, and hencetheir average velocity is higher, and it follows that although theytravel a greater distance to the aperture, and through a stronger totalmagnetic field, they none the less may be brought to a focus at theaperture as accurately as those traveling axially.

The concave cathode has a like effect, tendin to equalize the length ofpath of the electrons to the aperture.

In practice, I prefer to utilize both effects to obtain the optimumcorrection. The flatter the cathode, the shallower should be the cupformed by cathode film i8, and the deeper the anode cup, and vice-versa.The mathematics of design is tedious rather than difllcult, and thereare a large number of solutions giving substantially equivalent results.The figure is proportionally flection. The important point is that thescan tiplication would be of the order of 10- amperes or even less, to aminimum of 1 electron. 3 Such small currents normally require atremendous amount of exteriorampliflcation before they are of sufficientmagnitude to 'be of practical use,

and my present invention therefore is of great importance in that thesecurrents may be greatly increased within the dissector tube itself sothat a great reduction in outside amplification can be used.

One of the main objections to such extensive outside amplification isthat the noise level becomes excessively high, but by the use of my '75present invention, the signals alone are multiplied without noise sothat the signal-to-noise ratio is maintained at a high value. Theremainder of discussion, therefore, will have to do with the action ofthe multiplier itself and as this action will be the same irrespectiveofwhere electronsto make the complete multiplier assembly as small aspossible. I therefore prefer to give speciiic measurements of oneparticular multiplier which has been in use for purposes as describedlator M which may have any frequency up to 30 above. The space enclosedby the silver cathodes 2i and 22 has a diameter of of aninch. The anodeii is an axial .010 inch tungsten wire and the silver resonating coil 32is of a size which will resonate the cathodes at approximately 200megacycles. The resonating coil 32 is excited by the output of anexciting oscillator 49 which of course is turned to the resonantfrequency of about 260 megacycles. Only a single wire is needed tocomplete the R. F. circuit to the resonating coil because of thecapacity between the multiplier cathode 2i and the anode sleeve it, thelatter being grounded; This 200 megacycle oscillator may conveniently bea vacuum tube oscillator or even in itself a modification of theFarnsworth electron multiplierwhich is capable of sustaining selioscillations, which is described by me elsewhere.

In the event that a thermionic tube oscillator oi the usual type isused, I prefer to supply the oscillator with about 3 or ye oi thenecessary anode voltage through an inductance it which is coupled to alow frequency interrupting oscilmegacycles and even as low as 80 cycles,if desired for specific P rp ses.

The adjustment of the amount of multiplication occurring between the twocathodes 2i and 22 may be conveniently made by varying the voltage ofthe anode source 52 which supplies the low frequency oscillator. Theremainder of the voltage for the high frequency oscillator is suppliedby a high frequency anode supply source 54. when the high frequency andloan irequency oscillators are both operating, thecathodes Hand 22 arealternately and intermittently excited and a multiplier anode voltage issupplied to the anode Ii by a steady multiplier anode source 55.cathodes 2i and 22 and the central anode ll constitutes an electronmultiplier wherein electrons are repeatedly oscillated between the twocathodes at a velocity suilicient to create secondaries on impacttherewith, certain 0! these electrons being collected by the anode H;the low frequency oscillator intermittently and periodicallyinterrupting the energization of the cathodes.

The electrons which initiate the multiplication enter the cathodeaperture ll and the multiplication' which takes place withinthe cathodeenclosure is of course dependent, other factors remaining the same, uponthe-number of electrons entering the chamber. The multiplied electronscollected by anode Ii will be in proportion to the number'enterlng theaperture 30 up to the point,

might be caused by the' caesium ions contacting Thus, the combination ofthetwo' However, due to the fact that the low frequency oscillatorinterrupts the action of the high frequency oscillator periodically,these limiting factors are not able to persist and the multiplication 8can be carried on a great deal further. The output is taken fromthe-anode l l as a potential generated by currents flowing through anoutput resistor 56 and conducted for further use by an output connection51. v 4

Certain other-factors sometimes enter, the pic'- ture. It can be seen byan examination of Figure 1 and from the discussion as above, that thesecondary emission surfaces are preferably formed by the deposit ofcaesium thereon. There is a very definite limitation imposed bythepresence oi caesium ions. The caesium ions will be attracted towards thenegative cathode and when they impact the cathode, they too will createsecondaries. If these secondaries happen'to be in phase with theelectrons already oscillating between the two cathodes, the result isnot harmful. It is not usual, however, for the phase to be the samebecause the positive ions have a mobility of almost exactly 36 that ofthe electron. By closing the end of the multiplier structure as shown,no caesium ion will have to travel more than one-half the diameter ofthe cylindrical cathodes. Furthermore, any caesium ions which are closeto the anode are in a relatively intense 00 field and will thereforemake the trip from anode to cathode in no longer than 150 times the timerequired for an electron to travel between the cathodes. It can beexpected, therefore, that the caesium ions can be cleaned out of thetube in a time equal to 150 times the one-half period of the cathodefrequency of about 200 megacycles.

In practice, I have found that this time is ample to completelyeliminate any hcldover action which the cathodes.

While I have described the multiplier as being used with a 200 megacycleexciting oscillator, it is manifest that the exciting oscillator can beused to provide a lower frequency and oi course if this is done, theinterrupting oscillator should a be dropped in frequency, accordingly.The use,

, however, in this particular case, of the 200 megacycle oscillator,makes possible the use of a very small multiplying structure andsufllciently small 50 for incorporation in the dissector tube as shown,without substantial light obstruction.

In this particular case, no external magnetic focusing field isnecessary in conjunction with the cathodes as the shape of the cathodestou gether with the axial position of the collecting anode gives anelectrostatic held when energised, within the space enclosed by thecathodes. of the proper shape to permit good multiplication.

with the particular arrangement as shown and .9 described, suitablegains of 100,000 to 1,000,000 are easily obtainable. Experimentally, acurrent 7 input of 10- ampercs gave an output current of from-.1 to 1millilunpere.v -'.[he,muitipller itselfpasses a very small current. notmore than a microampere with no input electrons. The resonatinginductance is preferably incorporated inside the dissector close to thecathodes because this pennits'more ofiicient handling of the 200megacycle frequency.

The useof additional battery voltage to supply. the anode of the'200megacycle oscillator is of course not strictly necessary, but cconomiscson the power required in the lowfrequency oscilla- 5 where the limitingfactors above t? in V tor. The riot result of the incorporation of the7g multiplier of this type in the dissector tube reduces the amount ofgain required outside the dissector tube to a maximum of perhaps 100,-

There are a number of other applications of the method of periodicallyinterrupting the action of anelectron multiplier, which are shown inFigures 4 to 8 inclusive.

In these figures, circuits are shown whereby an electron multiplier isused for detection of radio frequency signals. It will be noticed thatin all of these three circuits, the radio frequency, presumablymodulatedor keyed in accordance with signals, is the sole energizationof the oathodes. In other words, a multiplier tube such as has beendescribed, is used without any R. F.

excitation of the cathodes except the signal. The amount of signalnecessary to operate a tube in this manner depends primarily on howsensitive the cathode surfaces are and how efflcient the transfer ofelectron energy through the circuit is. As the efiiciency of these twofactors is increased, the tubes sensitivity increases until final-' lyit will become a good self-oscillator. Tubes, however, which will notself-oscillate in a circuit as shown, for example, in Figure 4, wherethe cathodes are energized solely by an incoming R. F. signal train,'work well as a detector with a signal of the order of .1 volt or less.

For example, if the device is to be used as a straight multiplier, itmay be constructed with a pair of opposing cathodes G and BI with a ringanode 62 positioned between them. A tuned circuit N, comprising aninductance and capacity has its opposite ends connected to the cathodesand its midpoint 85 grounded. The tuned circuit is fed from a primaryinductance '66 which may be in the output of a radio frequencyamplifier, or connected directly to an antenna system comprising anaerial and a ground ,69 or equivalent collector. If, then, the ringanode 2 were to be "energized directly from an anode battery, forexample, at a voltage of 70 volts, the R. F. voltage across the cathodeswould be tremendously increased due to the multiplication created by theelectrons within the tube, oscillated under the-influence oi the appliedR. F. voltage. The time of flight within the tube may be convenientlyadjusted by varying the voltage of the anode battery for the particularwavelength being received, so that the time oi flight corresponds atleast in some degree to the incoming frequencies. The device operatingunder these conditions is of course supplied with its original electronseither by beginning the multi- I, therefore, prefer to interrupt theanode supply at an intermediate frequency, preferably 2x10 cycles when50 to 150 megacycles are being received. The interrupting frequency iscontrolled by the tuned circuit 10 connected to the anode and fed at thedesired frequency by any suitable oscillator. Inasmuch as the multipliertube illustrated in Figure '4 is provided with parallel planar plates,it is desirable to place the tube under the influence of an externalmagnetic field as indicated by arrows II in order that the multiplyingaction may be eflicient.

By the use of the interrupting intermediate frequency, much larger gainsmay be obtained in the output of the multiplier and it can be readilyseen by those skilled in the art that the receiving circuit of Figure 4will be suitable for use with a steady anode supply under certaincircumstances, and suitable for other uses with an interrupted anodesupply as shown in the drawings, the tube operating in both cases inidentical .manner as regards the energization of the oathodes directlyfrom the incoming signal.

It should also be pointed out that when an interrupted anode supply isused that the detected component may be obtained directly from the anodecircuit or the output may be taken oil? at the interrupting frequency ifit is desirable to further amplify with an intermediate frequencyamplifier. Thus, the multiplier device connected as shown in Figure 4may be used as a primary oscillator, or at least a device acting as afrequency converter, where intermediate frequency amplification mightappear desirable.

The same results can be obtained with a detector circuit as shown inFigure 5. Here signal energy contained in the primary 66 of the R. F.transformer is transferred to the tuned circuit 84 having its midpoint65 grounded, and this energy is then led to the cathodes and is the solesource of potential for these cathodes. The anode in this case ispreferably an axial rod 12 having wound around it and connected theretoa fairly wide mesh grid II. In this case, no external field is necessaryfor focusing the electrons because the two plates are semi-cylindricaland the static field created by their apposition is sufiicient toprevent immediate collection. the anode with a grid connected to it inorder that the probability of collection may be increased. In otherwords, electrons entering the space bounded by the grid wires will bemore probably collected due to the shielding action of the grid.

It is of course obvious that electrons passing through short chords ofthe grid space will not be collected, while those passing through longerchords approaching the diameter will be collected. By shaping thecathodes, I am able to decrease the probability of collection because ofthe shaping of the electrostatic field therebetween and by putting agrid around the anode I am able to increase the probability ofcollection, both the increase and decrease of probability by the twomethods being independent of one another. Thus, I am able to regulatethe probability, for example, of collection without interfering with thestatic field and I am able to change the static field and compensatetherefor by the use of the grid to obtain certain desired results.

The circuit as shown in Figure 5 is adapted to be used as an oscillatingdetector and I have therefore shown a source of exciting- R. F. 15 forthe anode 12-14. This, excitation is not I use.

necessary, however, in case the multiplier tube associated with thecircuit is a self-oscillator. In other words, if the multiplier issufficiently sensitive to generate self-oscillations, the R. F. isunnecessary. Therefore, the multiplier shown in Figure 5 is not only avery goodregenerative detector, either with or without an excitingsource, according to its sensitivity, but is also capable of beingoperated as a super-regenerative detector interrupting itself at afrequency which will be determined by an inductance it placed in serieswith an output device '87 and the variable source E8, the latter beingvariable in order to regulate the time of flight in accordance with theincoming signal. Whether or not there is an exciting R. F. supplied tothe tube, I prefer to have the device oscillated at to 100 megacyclesfor a signal frequency 'of '30 to 60 megacycles, these adjustments,however, being all within the skill of those familiar with the art.

This particular arrangement is extremely sensitive and a satisfactorydetector. Its high sensitivity is obtainable without criticaladjustment.

Here again, the detected component may be obtained directly from theanode circuit, or the output may be used to supply an intermediatefrequency amplifier. The interrupting action produced by the resonantcircuit in series with the anode is obtainable either on the portion ofthe multiplier characteristic showing a negative resistance or at adifference of frequency between the electron period and the signalfrequency, or at a difference of frequency between the exciting R. F.and the signal frequency, as may be desired.

It is also possible and sometimes preferable to build the multiplier asa photo-ionic tube duplicating the action of the tube shown in Figure 5.Such a tube and circuit is shown in Figure 7. In this combination theinterrupting action is obtained by beat between the signal frequency andthe electron frequency. Here, again, the cathodes are supplied solely bythe signal circuit Sit-5%, while the anode in this case is preferably arelatively close meshed grid 19. Inside the grid is positioned a heatedfilament which is, however, not adapted to provide electrons by emissiontherefrom, but is purely a source of light so that the photosensitivecathodes may be initially energized to emit photoelectrons. This isoperable because practically all surfaces readily emitting secondariesupon impact are also photosensitive. In this way, the number of tripsnecessary to build up the multiplier current is reduced. The tunedcircuit 10 is attached as usual in the anode circuit comprising theoutput device 17 and the anode supply 18, and an oscillator may becoupled thereto to provide interruption.

I have thus provided a means and method whereby the output of amultiplier tube may be greatly increased by the removal of certainlimiting factors; principally by interrupting the action of themultiplier. The multiplier may be either an oscillating or anon-oscillating condition. It may be supplied with a varying source ofelectrons, with a steady source of electrons, or with no source at all,reliance being placed in the latter case on casual electrons present. Imay prefer to deliberately interrupt the action or to so connect thetube that it will interrupt itself. The latter condition can beaccomplished when the tube is sufficiently sensitive to be aselfoscillator. I have shown that either thecathode energization or theanode potential may be interrupted. I have shown that tubes interruptedin this manner may be used to multiply an extraneous source of electronsvarying in number, or I may utilize the interrupting action tofacilitate the use of the tube as a detector, the output of the detectorbeing available both as a detected component, or as an intermediatefrequency carrying the signal impulses. And I have further shown thatthe multipliers may be used without any external guiding field and whenused as a detector may have the cathodes energized solely by a modulatedR. F. preferably one which is derived from space.

I have further shown that I may regulate the probability of collectionin two manners: 1) by the regulation of the fields through which theelectrons travel, due to the arrangement of the cathodes, or an externalfield; or (2) I may place means around the anode in order to provide asubstantially equipotential space of relatively large diameter aroundthe anode without substantial mechanical obstruction to the flight ofelectrons.

I claim:

1. An electron multiplier comprising an envelope containing a pair ofopposed electrically separate cathodes together describing a cylinder,

portions of said cathodes being extended transversely to substantiallyclose the ends of said cylinder, and an axial anode therebetween.

2. An electron multiplier comprising an en velope enclosing a pair ofopposed cathodesurfaces, a collecting electrode between said surfacesand a resonating coil within said envelope connected at each end to oneof said cathodes.

3. An electron multiplier comprising an envelope enclosing a pair ofopposed silver electrodes, a collecting electrode between said surfaces,and a'silver resonating coil within said envelope directly connected ateach end to one of said surfaces.

PHILO T. FARNSWORTH.

